In laser direct-write (LDW) techniques, a laser beam is used to create a patterned surface with spatially-resolved three-dimensional structures by controlled material ablation or deposition. Laser-induced forward transfer (LIFT) is an LDW technique that can be applied in depositing micro-patterns on a surface.
In LIFT, laser photons provide the driving force to catapult a small volume of material from a donor film toward an acceptor substrate. Typically, the laser beam interacts with the inner side of the donor film, which is coated onto a non-absorbing carrier substrate. The incident laser beam, in other words, propagates through the transparent carrier before the photons are absorbed by the inner surface of the film. Above a certain energy threshold, material is ejected from the donor film toward the surface of the substrate, which is generally placed, in LIFT systems that are known in the art, either in close proximity to or even in contact with the donor film. The applied laser energy can be varied in order to control the thrust of forward propulsion that is generated within the irradiated film volume. Nagel and Lippert provide a useful survey of the principles and applications of LIFT in micro-fabrication in “Laser-Induced Forward Transfer for the Fabrication of Devices,” published in Nanomaterials: Processing and Characterization with Lasers, Singh et al., eds. (Wiley-VCH Verlag GmbH & Co. KGaA, 2012), pages 255-316.
LIFT techniques using metal donor films have been developed for a variety of applications, such as repair of electrical circuits. For example, PCT International Publication WO 2010/100635, whose disclosure is incorporated herein by reference, describes a system and method of repairing electrical circuits in which a laser is used to pre-treat a conductor repair area of a conductor formed on a circuit substrate. The laser beam is applied to a donor substrate in a manner that causes a portion of the donor substrate to be detached therefrom and to be transferred to a predetermined conductor location.
As another example, U.S. Patent Application Publication 2011/0097550 describes a method of depositing a material on a receiving substrate. The method comprises providing a source substrate having a back surface and a front surface, the back surface carrying at least one piece of coating material. A receiving substrate is positioned adjacent to the source substrate and facing the coating material. Light is radiated towards the front surface of the source substrate, to remove at least one piece of the coating material from the source substrate and deposit the piece onto the receiving substrate as a whole. In accordance with an exemplary embodiment of the invention, the produced receiving substrates serve as solar cells, and a solar flat panel may be produced by connecting electrically multiple cells.
Another laser-based method for metallization of solar cells is described by Wang et al., in an article entitled “Silicon solar cells based on all-laser-transferred contacts,” published in Progress in Photovoltaics: Research and Applications 23 (2015), pages 61-68. Crystalline silicon solar cells based on laser-transferred contacts were fabricated with both front and rear metallization achieved through laser-induced forward transfer. Both the front and rear contacts were laser-transferred from a glass slide coated with a metal layer to the silicon substrate already processed with emitter formation, surface passivation, and antireflection coating. Ohmic contacts were achieved after this laser transfer.